The invention relates to the reinforcement of the walls of underground tunnels and rock chambers by prestressing. It furthermore relates to a method of making the prestressed cavity walls impermeable against seepage of liquids.
In tunnelling and in excavating underground rock chambers one of the main problems is the displacement of the tunnel walls by the enormous weight of the overlying rock and soil masses, often making bracing and lining imperative in order to prevent collapse of the excavation. This is not always necessary with hard rock where large sections of a tunnel may be self-supporting; however in deep-lying sections the very high pressure reached, causes plastic flow in the rock with subsequent displacement of the cavity walls; to counteract this pressure and to prevent penetration of water, internal lining -- made of concrete in modern structures -- is applied to the walls. In soft rock, such as lime stone, tunnelling generally requires a permanent lining, especially where fissures can be expected and the overlying weight is large.
Theoretically, a cylindrical or spherical cavity within a rock mass would not collapse under the weight of the mountain, since equally distributed stresses should appear according to simplified calculations based on the classical theory of elasticity. In practice, however, the forces acting along the circumference are not uniformly distributed, but may appear as alternate compression and tension zones in different portions of the wall. Rock can withstand considerable compression forces, unless the stress exceeds the plastic flow limit, but its tensile strength is low, so that tension or shearing stresses may cause cracks and fissures and eventual caving-in of the walls.
Lining, therefore, becomes necessary in tension zones; however since these cannot be clearly defined and detected during excavation, the whole tunnel length is generally provided with lining which incidentally controls water seepage. The same rules apply to the construction of underground chambers and tanks, be they of spherical, ellipsoidal or other vaulted shape.
In certain cases, where only a clearly defined portion of the tunnel or cavity shows fissures or signs of collapse, it is customary to insert and anchor long bolts into holes drilled into the rock, with the aim to strengthen the affected portion and to hold the rock in place. In the plurality of cases, however, because of the unreliabitity of a partly reinforcement only, complete lining of the area is greatly preferred.
Lining of any rock cavity is expensive per se, whether it is in the form of steel casings, prefabricated concrete sections or cast-in-situ concrete. It causes additional expenses by making necessary the excavation of the volume of rock taken subsequently by the lining and, in tunnelling, slows down the excavation progress by clogging up the narrow tunnel section owing to the large amounts of building material to be transported.
In view of these drawbacks it has been made the object of the present invention to dispense with the lining of tunnels or other excavations altogether and to replace it by converting the rock around the cavity into a self-supporting and load-bearing structure, by prestressing it to a degree beyond the largest tension and shearing stresses apt to occur in the respective portion of the cavity wall.